Often applied in automation technology, especially in process automation technology, are field devices, which serve for registering and/or influencing process variables. A field device, is, in such case, especially selected from a group composed of flow measuring devices, fill level measuring devices, pressure measuring devices, temperature measuring devices, limit level measuring devices and/or analytical measuring devices.
Flow measuring devices include especially Coriolis-, ultrasonic-, vortex-, thermal and/or magneto inductive, flow measuring devices. Fill level measuring devices include especially microwave fill level measuring devices, ultrasonic fill level measuring devices, time domain reflectometric fill level measuring devices (TDR), radiometric fill level measuring devices, capacitive fill level measuring devices, inductive fill level measuring devices and/or temperature sensitive fill level measuring devices. Pressure measuring devices include especially absolute-, relative- or difference pressure measuring devices. Temperature measuring devices include especially measuring devices with thermocouples and temperature dependent resistors. Limit level measuring devices include especially vibronic, ultrasonic, limit level measuring devices and/or capacitive limit level measuring devices. Analytical measuring devices include especially pH-sensors, conductivity sensors, oxygen- and hydrogen peroxide sensors, (spectro)-photometric sensors, and/or ion-selective electrodes.
A large number of such field devices are produced and sold by the firm, Endress+Hauser.
High-frequency apparatuses, such as, for example, field devices, are composed of a number of components, which work with an operating frequency or with fractions of the operating frequency. For example, the radar based Micropilot ES FMR50 fill-level measuring device of the company Endress+Hauser works with an operating frequency of 25.5 GHz. The working frequencies of the individual components of this fill-level measuring device are 25.5 GHz, 12.75 GHz, 750 MHz or 1.5 GHz, 300 MHz and 140 kHz.
Especially with increasing frequencies, reflections occur in the connecting lines between the individual components. Dependent on the radar measuring principle utilized (pulse radar, modulated continuous wave radar (so-called FMCW), Doppler radar, etc.), the reflections can influence accuracy of measurement, measurement linearity, resolution and maximum measured distance. Moreover, in the case of pulsed signals, the pulse shapes are influenced by reflections. From this there results in the case of a defined pulse width of the transmitted signal pulses a greater pulse width, which can interfere with the neighboring signal pulse. The interfering of a signal pulse with a neighboring signal pulse influences measurement error, which becomes more noticeable with greater distances between transmitter and receiver of the signal pulses.
There are methods for reducing reflections in connecting lines between two components of a high-frequency apparatus. In radar systems for fill level measurement, usually the line impedance of two connected components is fixed, in each case, at 50 ohm. In such case, only the ohmic resistances and not the reactances of the components are considered.
If two components are connected with one another by means of a corresponding connecting line, purely theoretically, no reflection occurs, because of the equal ohmic resistances of the two components. In the real case, however, components always have a reactance, which must be taken into consideration for preventing reflections in the connecting line.
This taking into consideration is accomplished in the method of impedance matching. In such case, the impedance of the first component is set equal to the complex conjugated impedance of the second component. The impedances are complex valued and are composed of a real- and an imaginary part, which are both frequency dependent. The imaginary part represents, in such case, the travel time of a reflection at the respectively considered frequency. By impedance matching, reflections between two components are optimally suppressed. Problematic here are, for example, tolerances of components and component inaccuracies in batch production.
The method of impedance matching is suitable not only for the given operating, respectively working, frequencies, but, instead, also for neighboring frequencies. In this way, a frequency band results, for which the impedance matching approximately holds. Especially, radar devices working according to the frequency modulated continuous wave principle (FMCW) have a frequency band. Pulse radar devices have a frequency band with a center frequency, wherein the highest power density is at the center frequency. Frequencies around the center frequency must, consequently, especially be taken into consideration, in order to achieve impedance matching for the entire frequency band.
Impedance matching can be implemented by means of adapting structures placed on the connecting line. If an impedance matching is to be approximately true for an entire frequency band, this leads to adapting structures on the connecting line, which in the total frequency band transmit the signal pulses with a uniform group travel time. In this way, dispersion effects are prevented in the signal pulses. In the case of the application of a continuous wave signal (FMCW), this plays only a lesser role.
Known from the state of the art is a method for lessening reflections, in the case of which a damping mat is adhered to a circuit board integrated, high-frequency, connecting line. Such damping mats are offered under the mark “Eccosorb”. More recently, materials with similar properties are also available in the form of paste. A disadvantage of a damping mat is undesired signal loss. This is especially disadvantageous in the case of 4-20 mA devices. Furthermore, there is in the case of pulse radar systems an upper limit for the attenuation according to the correlation principle (so-called dynamic range), which shows on the correlator as increased noise level and lessened amplitude of the wanted echo. Furthermore, most components have a number of level states, each having a different impedance.